Monolithically integrated signal processing circuit having active and passive components

ABSTRACT

An optical signal processor is implemented as a monolithically integrated semiconductor structure having optical waveguide devices forming beam splitters, optical amplifiers and optical phase shifters. The monolithic structure photonically controls a phased-array microwave antenna. Phase-locked master and slave lasers generate orthogonal light beams having a difference frequency that corresponds to the microwave carrier frequency of the phased-array antenna. The lasers feed the signal processor, which performs beam splitting, optical amplifying and phase shifting functions. A polarizer and an array of diode detectors convert optical output signals from the signal processor into microwave signals that feed the phased-array antenna. The optical waveguides of the signal processor are fabricated in a single selective epitaxial growth step on a semiconductor substrate.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold, importedand licensed by or for the Government of the United States of Americawithout the payment to us of any royalty thereon.

This is a Divisional Application of application Ser. No. 08/709,997which was filed Sep. 9, 1996 and issued as U.S. Pat. No. 5,770,472.

FIELD OF THE INVENTION

This invention relates generally to the field of micro-electronics andmore particularly to the monolithic integration of opto-electronicsignal processing structures for use in photonic systems.

BACKGROUND OF THE INVENTION

In recent years, artisans have made significant advances in fabricatingand using opto-electronic integrated circuits. These improved circuits,which often contain passive and active optical devices, have foundsignificant applications in a number of fields including opticalcomputing and communications. The use of opto-electronic circuits inmany systems can result in significant cost savings, increased circuitspeeds, reduced physical size and power requirements, increasedreliability, as well as other improvements. As such, those concernedwith the development of photonic systems in such fields as radar,communications and computing have recognized the need for improvedtechniques of constructing opto-electronic circuits.

Specifically, conventional phased-array antenna systems have beensuccessfully demonstrated for transmitting and receiving microwaveenergy in communication and radar systems. A phased-array antenna is anantenna with two or more driven elements. The elements are fed with acertain relative phase, and they are spaced at a certain distance,resulting in a directivity pattern or beam that exhibits gain in somedirections and little or no radiation in other directions. Althoughphased arrays may have fixed beams, they usually contain rotatable orsteerable beams. Of course, an antenna structure may be physically movedto effect beam steering, however, in a phased-array antenna, beamsteering is usually accomplished by simply varying the relative signalphase being fed to the antenna elements.

Although prior art phased-array antennas have served the purpose, theyhave not proved entirely satisfactory for use in many microwavecommunication and/or radar systems. To obtain sufficient radarresolution, phased-array antennas employed with some microwave radarsrequire that as many as a thousand antenna elements be arrayed toproduce a sufficiently narrow beam. Since many long-range communicationsystems also require narrow antenna beams, they often have antennaarrays with hundreds of antenna elements. While the size of microwaveantenna arrays having several hundred or even a thousand antennaelements can be relatively small, the signal processing circuitsconnected to these antenna arrays often become prohibitively large andexpensive to manufacture. Thus, artisans have recognized the need forreducing the size and cost of many signal processing circuits by usingan optically controlled microwave system.

SUMMARY OF THE INVENTION

A general purpose of this invention is to provide a technique offabricating a monolithically integrated circuit by growing a network ofinterconnected active and passive optical waveguide devices on asubstrate in a single selective epitaxial growth step.

A specific aspect of the invention comprises an optical signal processorfor optical phase distribution and microwave beam forming. A network ofinterconnected optical waveguide devices are built on a semiconductorsubstrate. The optical waveguide devices comprise optical beamsplitters, optical amplifiers and optical phase shifters. A gain controlcircuit connects to the optical amplifiers for amplifying optical energypropagating through the optical amplifiers. A phase-shift controlcircuit connects to the phase shifters for controlling the relativephases of two optical beams with orthogonal polarization.

A more specific aspect of the present invention comprises a microwavesystem having a photonically controlled phased-arraymicrowave/millimeter wave antenna. The system comprises an opticalsignal processor formed from a monolithically integrated semiconductorstructure having a network of interconnected waveguide devices. Thewaveguide network includes optical beam splitters, optical amplifiersand optical phase shifters. Phase-locked master and slave lasersgenerate two orthogonal light beams having a difference frequency thatcorresponds to the microwave/millimeter wave carrier frequency of thephased-array antenna. The lasers feed the optical signal processor whichperforms beam splitting, optical amplifying and phase shiftingfunctions. The optical signal processor has a plurality of opticaloutputs equal in number to the number of antenna elements. A polarizerat the outputs and an array of optical fibers transmit these opticaloutputs to diode detectors that generate microwave signals which feedthe phased-array antenna. The optical waveguides are fabricated in asingle selective epitaxial growth step on a semiconductor substrate.

The exact nature of this invention, as well as other objects andadvantages thereof, will be readily apparent from consideration of thefollowing specification relating to the annexed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a microwave system having aphotonically controlled phased-array antenna in accordance with thepresent invention.

FIG. 2 is a schematic block diagram of an optical signal processorcircuit for use in the system of FIG. 1.

FIG. 3 is a schematic diagram showing a top plan view of the FIG. 2circuit implemented as a monolithically integrated semiconductorstructure having a network of interconnected optical waveguide devices.

FIG. 4 is a schematic elevation showing a cross section of semiconductorlayers used to fabricate the network of optical waveguide devices ofFIG. 3.

FIG. 5 is a pictorial break-away schematic in cross section illustratingthe fabrication process used to construct the optical waveguide devicesshown in FIG. 3.

FIG. 6 is a top plan view in schematic illustrating an intermediate stepof the fabrication process used to construct the optical waveguidedevices shown in FIG. 3.

FIG. 7 is a graph of layer thickness vs. strip width useful inunderstanding the fabrication process of the present invention.

FIG. 8 is a graph of indium (In) fraction vs. strip width useful inunderstanding the fabrication process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 depicts photonic microwave system11 for transmitting and/or receiving microwave/millimeter wave radiationat frequency f3 via a photonically controlled phased-array antennasystem. Microwave system 11 comprises conventional master laser 12 andslave laser 13 perpendicularly polarized and phase-locked to each othervia line 14. Master laser 12 generates light at frequency f1 while slavelaser 13 generates light at frequency f2. Light frequencies f1 and f2are chosen such that their difference, (f1-f2), corresponds to thecarrier frequency f3 of microwave system 11. Master laser 12 launches alight beam in the TE mode onto optical waveguide 15, which transmits thelight beam to a conventional optical modulator 16. Modulator 16impresses information onto the light beam by, for example, pulsemodulation. As such, modulator 16 passes the modulated light beam, e.g.,pulses of light having carrier frequency f1 and polarized in the TEmode, onto optical waveguide 17. Synchronously with laser 12, laser 13launches a light beam having frequency f2 and polarized in the TM modeonto optical waveguide 18. Consequently, the outputs of waveguides 17and 18 simultaneously feed two phase-related orthogonal light beams tooptical waveguide 19. These orthogonal light beams form an input tooptical signal processor 20.

With further reference to FIG. 1, optical signal processor 20 splits theperpendicularly polarized light beams input by waveguide 19 into anumber of channels, amplifies the light intensity in each channel andphase shifts the light in each channel. Beam steering circuit 23, whichinputs beam steering voltages (v_(i)) via bus 39 to optical signalprocessor 20, functions to vary the amount of phase shift in each of thechannels i, where i represents an arbitrary channel. A conventionaloptical polarizer 21, polarized at a 45-degree angle with respect to thepolarization of the orthogonal light beams, mounts at the output ofoptical signal processor 20. Polarizer 21 extracts and combines45-degree light components from each channel. Thus, polarizer 21produces polarized light containing the sum and difference frequenciesf4 and f3, where f1+f2=f4 and f1-f2=f3, as well as frequencies f1 andf2. An array of optical fibers 22, one fiber for each channel, transmitslight energy from the output of polarizer 21 to an array of opticaldiode detectors 25, each of which oscillate at frequency f3, i.e., thedifference frequency (f1-f2), which corresponds to the carrier frequencyf3 of phased-array antenna 24. The respective output lines 27 of diodedetectors 25 feed different ones of the specifically spaced antennaelements of phased-array antenna 24 with microwave signals at carrierfrequency f3 but with different relative phase angles.

As mentioned above, optical signal processor 20 performs beam splitting,amplifying and phase shifting functions. FIG. 2 depicts a functionalblock diagram of optical signal processor 20, which is designed togenerate (n×m) output signals that feed the (n×m) antenna elements ofphased-array antenna 24. The input stage of optical signal processor 20includes a (1×n) beam splitter 30, which feeds n polarization-dependent,light-intensity amplifiers 31. Input bias voltages 29 control amplifiers31 via bus 38. The n outputs of amplifiers 31 connect to an n(1×m) beamsplitter 32, which in turn feeds an array of (n×m)polarization-selective, phase shifters 33. Beam steering voltages(v_(i)) from beam steering circuit 23 control phase shifters 33 via bus39. The (n×m) outputs of phase shifters 33 pass to polarizer 21 (seeFIG. 1). For a typical microwave phased-array antenna having 256 antennaelements, the values of n and m may each be equal to sixteen. Of course,phased-array antennas having other numbers of elements and othercombinations of n and m are of equal importance and will become obviousto those skilled in these arts.

FIG. 3 is a schematic drawing of an illustrative optical signalprocessor 20 where, for simplicity, the values of n and m both equalfour. Consequently, FIG. 3 shows a single input channel and sixteenoutput channels. Optical signal processor 20 is implemented as amonolithically integrated semiconductor structure comprising a networkof optical waveguide devices which function as beam splitters 30 and 32,optical amplifiers 31 and optical phase shifters 33. Beam splitter 30comprises a series of passive optical waveguides connected in a treeconfiguration and fed by optical waveguide 19. The input waveguide ofbeam splitter 30 branches into two waveguides that continue to branch inlike fashion, forming four output waveguides that feed fourpolarization-dependent, light-intensity amplifiers 31. Amplifiers 31 arewaveguide devices that amplify light energy as it propagates through theamplifiers. Bias voltages 29 control the gain of amplifiers 31.

Beam splitter 32, functionally similar to beam splitter 30, connects tothe outputs of optical amplifiers 31. Beam splitter 32 comprises passiveoptical waveguides in a tree configuration that grows from four inputchannels to sixteen channels. Thus, beam splitter 32 splits the fouramplified light beams output by amplifiers 31 into sixteen similar lightbeams, each of which comprise in-phase orthogonal light beams havingrespective frequencies f1 and f2.

Finally, the sixteen output channels of beam splitter 32 each connect toa different one of the sixteen phase-shifters 33. Beam steering circuit23 (see FIG. 1) transmits a set of sixteen beam steering voltages(v_(i)) to phase shifters 33 via bus 39. Beam steering voltages (v_(i))selectively phase shift each of the light beams propagating throughphase-shifters 33 such that the relative phases of the sixteen outputsof optical signal processor 20 produce a beam pattern having theappropriate directivity at phased-array antenna 24 (see FIG. 1).

In particular, at the inputs to phase shifters 33, the light beamspolarized in the TE mode, i.e., at frequency f1, may be expressed asA=A₀ sin(ω₁ t+.o slashed.₁); likewise, the light beams polarized in theTM mode, i.e., at frequency f2, may be expressed as B=B₀ sin(ω₂ t+.oslashed.₂). In these expressions, A and B represent the light intensityat frequencies f1 and f2, respectively, ω represents angular velocity, trepresents time and .o slashed. represents the initial phase angles.Phase shifters 33, being polarization sensitive, will, in general, phaseshift the perpendicularly polarized beams differently. Applied voltage(v_(i)) shifts light polarized in the TE mode in channel i a greateramount than light polarized in the TM mode in channel i. The followingexpression describes the phase shift of light polarized in the TE modein channel i:

    A.sub.i =A.sub.0 sin ω.sub.1 t+.o slashed..sub.1 +α.sub.1 (v.sub.i)!.sub.i,                                         (1)

where α₁ (v_(i)) represents the phase-shift angle due to beam steeringvoltage (v_(i)), and subscript i identifies the particular channel.Likewise, beam steering voltage (v_(i)) phase shifts light polarized inthe TM mode, i.e., at frequency f2, in channel i as follows:

    B.sub.i =B.sub.0 sin ω.sub.2 t+.o slashed..sub.2 +α.sub.2 (v.sub.i)!.sub.i,                                         (2)

where a₂ (v_(i)) represents the phase-shift angle for frequency f2 dueto voltage (v_(i)) from beam steering circuit 23. Due to thepolarization sensitivity of phase shifters 33, α₁ (v_(i)) will generallybe different from and much greater than α₂ (v_(i)). When polarizer 21and diode detector 25 combine light beams A_(i) and B_(i), an outputsignal having a difference frequency is generated in the classicalmanner. In particular, the output signal from diode detector 25 has acomponent at the difference frequency, which may be expressed asfollows:

    K cos  (ω.sub.1 -ω.sub.2)t+.o slashed..sub.1 -.o slashed..sub.2 +α.sub.1 (v.sub.i)-α.sub.2 (v.sub.i)!         (3)

where K is a constant. Expression (3) corresponds to the light energy inchannel i at the difference frequency (f1-f2), which equals frequency f3and corresponds to the output of detectors 25 and the microwave carrierfrequency of phased-array antenna 24.

In expression (3), the term .o slashed.₁ -.o slashed.₂ +α₁ (v_(i))-α₂(v_(i))! represents the phase angle for light energy in channel i at thedifference frequency f3 after being shifted by beam steering voltage(v_(i)). Since lasers 12 and 13 are phase locked, .o slashed.₁ and .oslashed.₂ are substantially equal, making their difference, (.oslashed.₁ -.o slashed.₂), nearly equal to zero. Also, since α₁ (v_(i))and α₂ (v_(i)) are generally different and α₁ (v_(i)) can be made verylarge compared to α₂ (v_(i)), beam steering voltage (v_(i)) can producearbitrarily large phase shifts in the resulting phase angle .o slashed.₁-.o slashed.₂ +α₁ (v_(i))-α₂ (v_(i))!.

Output signals from diode detectors 25 feed corresponding antennaelements of phased-array antenna 24. Due to the various phase shifts, α₁(v_(i))-α₂ (v_(i))!, in each of the sixteen channels, phased-arrayantenna 24 radiates at microwave carrier frequency f3 with a directivitypattern or beam that can be steered by beam steering voltages (v_(i)),where i ranges from one to sixteen.

As mentioned above, optical signal processor 20 is formed as amonolithically integrated semiconductor device. FIG. 4 schematicallydetails representative materials that may be used to construct thecomponents of optical signal processor 20. As seen in FIG. 4, thecomponents of optical signal processor 20 are waveguide devicescontaining the layers of a PIN diode. In particular, the components ofoptical signal processor 20 comprise layered structure 40 formed fromsemiconductor heterostructures epitaxially grown on n-type substrate 41,which functions as one of the PIN diode terminals. Using substrate 41 asa bottom cladding layer, a fabricator grows a few periods of a multiplequantum well (MQW). The MQW includes a series of un-doped barrier andwell layers 42 and 43, respectively. In a well known manner and beforegrowing layers 42 and 43, the fabricator may first improve the qualityof the upper surface of substrate 41 by covering it with an un-dopedbuffer layer (not shown). Layered structure 40, in the FIG. 4illustrative example, comprises a three-period MQW. When tailoring aparticular layered structure 40, skilled fabricators will appreciatethat the number of MQW periods will depend on their particularapplication; one to 20 MQW periods would probably be suitable for mostapplications.

Next, the fabricator grows an un-doped top cladding layer 44 on theupper well layer 43. The fabricator then grows p-type cap layer 45 oncladding layer 44. Metal layer 46 is then deposited on cap layer 45.Metal layer 46, which functions as a PIN diode terminal, is shown indashed lines in FIG. 4 to represent that not all of the waveguidedevices contain this layer. In particular and as shown in FIG. 3, onlythe active waveguide devices, i.e., amplifiers 31 and phase shifters 33,which have bias voltages applied thereto, include metal layers 46.Finally, to electrically isolate the biased devices, amplifiers 31 andphase shifters 33, from the passive beam splitters 30 and 32, thefabricator etches narrow groves 50 (see FIG. 3) in p-type cap layer 45at the boundaries where the biased devices meet the passive devices.

Layered structure 40 is preferably formed as an indium-phosphorus (InP)based structure. More particularly and as shown in FIG. 4, layeredstructure 40 includes the following semiconductor layers: n-dopedindium-phosphorus (InP N+) substrate 41; un-doped InP barrier layers 42;un-doped indium-gallium-phosphate-arsenide (In_(x) Ga_(1-x) P_(y)As_(1-y)) well layers 43; un-doped InP top cladding layer 44; and p-typeInP cap layer 45.

The waveguide devices of signal processor 20 are preferably fabricatedby selective epitaxial growth using metal organic chemical vapordeposition (MOCVD), or metal organic vapor phase epitaxy (MOVPE), or gassource molecular beam epitaxy (MBE). The fabrication process of thepresent invention permits the fabricator in a single selective epitaxialgrowth step to tailor the band gaps and the strain of the variouswaveguide devices of signal processor 20. In particular, for signalprocessor 20 to function in the intended manner, those waveguidesections that function as amplifiers 31 must have a band gap that issmaller than the energy of the light to be amplified, those waveguidesections that function as phase shifters 33 must have a band gap that isgreater than the light energy and must have sufficient strain toincrease polarization dependence, and those waveguide sections thatfunction as beam splitters 30 and 32 must have a band gap that is muchgreater than the light energy. In accordance with the present invention,waveguide devices with the appropriate band-gap spacing can befabricated in a single selective epitaxial growth step.

FIGS. 5-8 show structures and graphs useful in understanding thefabrication process of the present invention. The selective epitaxialgrowth process of the present invention involves the use of two parallelmasking strips 51, preferably formed from silicon-dioxide (SiO₂),selectively placed on substrate 41 to define a gap, of uniform width(d), in the shape of the waveguide devices of signal processor 20 (seeFIGS. 3, 5 and 6). As illustrated in FIG. 6, masking strips 51 havewidths (w) that vary with the nature of the component being grown.

FIG. 6 illustrates a set of parallel masking strips 51 placed onsubstrate 41 in a Y-shaped configuration with a 31 gap of uniform width(d). Gap 31' of FIG. 6 defines a Y-shaped region on substrate 41 onwhich a corresponding Y-shaped portion of beam splitter 30 will begrown. Gaps 31' of FIG. 6 define two rectangular regions on substrate 41on which corresponding amplifiers 31 will be grown. Gaps 32' of FIG. 6define two rectangular regions on substrate 41 on which correspondingportions of beam splitter 32 will be grown. Similar masking strips 51are placed on substrate 41 in a similar manner to define the otherportions of signal processor 20, including phase shifters 33.

As mentioned above, while all such gaps, e.g., gaps 30', 31' and 32' ofFIG. 6, have a common width (d), the fabricator varies the width (w) ofmasking strips 51 to vary the nature of the layered structure 40 that isto be grown within the particular gap. FIG. 5 illustrates a typicalcross section of a layered semiconductor structure that was grown withthe use of masking strips 51. As seen in FIG. 5, no material grows onmasking strips 51 during selective epitaxial growth. However,semiconductor material does grow on the un-masked surfaces of substrate41. The presence of masking strips 51 during epitaxial growthsignificantly effects the growth rate and shape of the semiconductorlayers for the material grown immediately adjacent thereto. Maskingstrips 51 also effect the material composition of the adjacentsemiconductor layers. As is well know by those skilled in these arts,size, shape and material composition, as well as other growth conditionfactors, will usually determine the resulting band-gap configuration ofan epitaxially grown layered structure.

FIG. 7 graphically illustrates how the width (w) of masking strip 51effects the layer thicknesses for layered structure 40 (see FIG. 5).Likewise, FIG. 8 graphically illustrates how the width (w) of maskingstrip 51 effects the In mole fraction for the InGaAsP layer.

As shown in FIGS. 7 and 8, the In_(x) Ga_(1-x) P_(y) As_(1-y) quantumwell layers 43 will have increased well width and In fraction as thewidth (w) of masking strips 51 increases, which results in a decreasedquantum well band gap due to a reduction in quantum confinement energyand increased In fraction. However, for the InP material of barrierlayers 42, the In fraction does not change due to a wider masking strip51. Thus, using selective epitaxial growth, the fabricator tailors theband-gap profile of the waveguide devices of signal processor 20 bysimply controlling the widths (w) of masking strips 51.

For the particular materials shown in the FIG. 4 illustrative example,the In_(x) Ga_(1-x) P_(y) As_(1-y) well layers may be grown with(43%<x<63%, 0<y<0.4) such that their quantum well band gaps are about0.8 electron volts (eV) or 1.55 micrometers (μm) with some compressivestrain in those regions designated as phase shifters 33 and amplifiers31. InP material is used to fabricate for barrier layers 42 because theaverage effective optical index of the well layers 43 in the MQWstructure is bigger than that of top cladding layer 44 and the bottomInP cladding layer, viz., substrate 41. Alternatively, barrier layers 42may be fabricated as lattice matched InGaPAs barriers with a band gap ofabout 1.1 eV. As an alternative to providing a P+ cap layer 45, thep-type doping of layered structure 40 may also be formed by p-typedoping cap layer 45 and top cladding layer 44 such that the dopinggradually varies from being P+ on the top surface of cap layer 45 tobeing un-doped at the MQW interface between cladding layer 44 and thetop well layer 43.

As an illustrative example in which the operating wavelength is about1.55 μm, those masking strips 51 that form beam splitters 30 and 32 havevery narrow widths (w) ranging from zero to 2.5 μm. As such, thecorresponding beam splitters 30 and 32 will have very narrow quantumwells of about 20-50 angstroms (Å) with a small In composition,producing a band-gap energy much greater than the operation wavelengthand with some small tensile strain in the well. Thus, beam splitters 30and 32 will be transparent to the propagating light and havesubstantially no light absorption therein.

Continuing with the illustrative example, the width (w) for thosemasking strips 51 that form phase shifters 33 are wider, e.g., 5 μm to15 μm, producing a wider quantum well layer 43 of from 70 Å-100 Å withcompressive strain. In this case, the band gap is about 20 meV-40 meV,higher than the operating light energy. Consequently, phase shifters 33can produce an effective polarization dependent phase shifting with theapplication of a reverse bias, beam steering voltage (v_(i)), due to thequantum confined Stark effect and larger heavy- and light-holeseparation by both quantum confinement and strain.

Still further, the width (w) of those masking strips 51 that formamplifiers 31 are even larger than those that form phase shifters 33.The resulting thicker well layers 43 with richer In compositionsdecreases the band gap to a value just below the 1.55 μm wavelength ofthe operating light energy. Thus, light intensity amplification can beachieved by current injection upon the application of a forward biasvoltage 29 to amplifiers 31. The optical gain will be polarizationdependent due to the strain and quantum confinement. In particular,there will be more gain for the TE mode for the system.

Consequently, the optical waveguide phase shifters 33 have enhancedpolarization selectivity due to the compressive strained quantum wells.This represents an improvement over existing phase shifters, whichnormally do not have enough polarization selectivity for near band-gapoperation. Additionally, since the present fabrication process comprisesa single selective epitaxial growth step, the resulting structure can beof high quality. The present one-step fabrication process, having nointermediate regrowth steps, is simple and inexpensive, producingmonolithically integrated structures of high quality with lessimpurities. The regulating layered compositions of structure 40 reducesstrain in most materials, e.g., thick cladding layers and barriers, andkeeps most tensile and compressive strains in the thin quantum welllayers 43, which should not exceed the critical thickness. As mentionedabove, these strains help produce the necessary polarizationselectivity. Consequently, the present fabrication technique can producemonolithically integrated signal processor 20 as an inexpensive,small-sized structure that eliminates many of the prior art problemscaused by the low efficiency of existing chip-to-chip interconnectionsand the unwanted impurities produced during existing integration byregrowth techniques.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. For example, while signalprocessor 20 of FIG. 3 has a relatively small number of waveguidedevices, for illustrative purposes, a more practical application of thepresent invention would involve a signal processor with hundreds orthousands of waveguide devices. In some applications, several sets ofamplifiers 31, placed at different beam splitting stages, might benecessary. Also, in other applications, amplifiers 31 and phase shifters33 may be moved from the linear portions of the waveguide network to thecurved and/or Y-shaped sections of beam splitters 30 and 32. Also,semiconductor materials other than those recited herein may also beused. For example, InP cap layer 45 may be replaced with a p-type,lattice matched layer of indium-aluminum-arsenide. Still further, theepitaxially grown material of layered structure 40, within the gap ofthe two oxide masking strips 51, may be tailored and/or etched tosupport various modes of waveguide operation. For example, the upperportion of layered structure 40 may be etched to a predetermined depth,thereby narrowing the width of the upper portion of layered structure40, for single mode light propagation. It is to be understood,therefore, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. An optical waveguide for processing lightsignals, comprising:a semiconductor substrate; a network ofinterconnected optical waveguide devices fabricated on said substrateusing a single selective epitaxial growth process, said opticalwaveguide devices including optical beam splitters, optical amplifiersand optical phase shifters, with each optical waveguide device beingfabricated in a gap of uniform width between masking strips and havingthe nature thereof determined by the width of the masking strips; gaincontrol means for amplifying said light signals propagating through saidoptical amplifiers; and phase-shift control means for controlling therelative phases of said light signals.
 2. The optical waveguide of claim1 wherein said substrate is a heavily doped layer and said opticalwaveguide devices each include multiple quantum well structures.
 3. Theoptical waveguide of claim 2 wherein said multiple quantum wellstructures include a series of barrier and quantum well layers.
 4. Theoptical waveguide of claim 3 further including a top cladding layer anda heavily doped cap layer mounted on said series of barrier and quantumwell layers, said substrate and said cap layer being doped with chargecarriers of opposite polarity.
 5. The optical waveguide of claim 4wherein said quantum well layers of said optical beam splitters have aband-gap energy that is much greater than the energy of said lightsignals.
 6. The optical waveguide of claim 5 wherein said quantum welllayers of said phase shifters have a band-gap energy that is greaterthan the energy of said light signals and said phase shifters arepolarization selective relation to phase shifters in another opticalwaveguide.
 7. The optical waveguide of claim 6 wherein said quantum welllayers of said amplifiers have a band-gap energy that is smaller thanthe energy of said light signals and said amplifiers are polarizationselective relation to amplifiers in another optical waveguide.
 8. Theoptical waveguide of claim 7 wherein said substrate comprises n-typeindium-phosphorous (InP), said barrier layers and said top claddinglayer comprise un-doped InP, said cap layers include p-type InP, andsaid quantum well layers include un-dopedindium-gallium-phosphate-arsenide.
 9. The optical waveguide of claim 8wherein said cap layers of said optical amplifiers and said opticalphase shifters have metal layers deposited thereon, and said gaincontrol means connects to said metal layers of said optical amplifiersand said phase-shift control means connects to said metal layers of saidphase shifters.